Water droplets moving through the flowpath of a steam turbine create at least two significant issues. First, the presence of water droplets in the steam turbine flowpath reduces stage efficiency. Second, such moisture causes premature erosion of the turbine blades or buckets, especially those in the last stage of the turbine.
Erosion of turbine blades in the last stages of a steam turbine is a common issue. In general, the erosion is caused by the combination of moisture, the high rotational speeds, and temperature values found in this area of the turbine. Conventional measures to prevent such turbine blade erosion and degradation, while costly, have proven to be largely ineffective. For example, certain measures focus on manufacturing the turbine blade to better withstand the extreme conditions within the steam turbine. The turbine blade, in its most affected areas (which generally include the leading edge of the tip of the turbine blade), is hardened during manufacture or a satellite shield is installed. While such efforts may stave off the erosion of the turbine blades for a short period, the erosion inevitably returns due to the extreme operating conditions within the steam turbine. Additional, such efforts add complexity and costs to the turbine blade.
Others protective measures have focused on the removal of moisture from the steam flowpath so that less water droplet moisture comes into contact with the turbine blades. Conventional systems employing this strategy generally focus on removing water through water drainage arrangements within the casing walls of the steam turbine or through suction slots made in hollow stator blades or nozzles. For example, some known systems attempt moisture removal using suction slots on the pressure or suction side of the nozzles. Other prior art systems focus on removing the moisture at a location that is in very close proximity to the tip of the turbine blade. Nevertheless, moisture in the steam turbine flowpath remains an issue. Thus, there is a need for improved systems for removing water in steam turbine engines.